Currently, no practical method exists for direct measurement of force production from individual muscles. Manual muscle tests do not give an accurate estimate of muscle strength. Measurements of joint torque are inadequate because several muscles often contribute to torque development. Implantation of a buckle transducer on a tendon is highly invasive and impractical for regular use. The electromyogram is the clinical standard used to measure the patterns and amount of electrical activity generated from resting and contracting muscle. However, the problem remains that electromyographic activity does not provide a quantitative measure of muscle tension. In contrast, our previous research has demonstrated that intramuscular pressure accurately measures both the active and passive tension of normal muscle. The overall objective of this project is to provide a useful clinical tool for in-vivo quantification of muscle force. We plan further development of the scientific foundation for use of intramuscular pressure as a powerful clinical tool. The specific aims of this study are to (a) continue development of a fiber optic microsensor to measure intramuscular pressure, (b) perform in-vivo human experiments to evaluate the relationship of intramuscular pressure to the recruitment of motor units, the number of active motor units, and the size of the compound muscle action potential, c) determine the relationship between intramuscular pressure and muscle tension for pathological electromechanical coupling conditions in an animal mode, and (d) guide clinical applications of intramuscular pressure using a continuum mechanics- based 3D finite element model of skeletal muscle. Successful development of this sensor will result in a powerful new clinical tool. The ultimate goal is to use this microsensor for clinical decision making and treatment of patients with neurogenic disorders (e.g. motor neuron disease, peripheral neuropathy), disorders of neuromuscular transmission (e.g. myasthenia gravis, Lambert-Eaton syndrome) and myopathies (e.g. muscular dystrophies, polymyositis, metabolic myopathies).